Hollow panel construction



Aug. 28, 1956 e. E. KLOOTE 2,760,240

HOLLOW PANEL CONSTRUCTION Filed May 26, 1950 2 Sheets-Sheet l @15 2 t: 2/ 25\ 25 Ailiimfl N "/l 7 III A mmmlml 32x:

Mme

Aug. 28, 1956 s. E. KLOOTE HOLLOW PANEL CONSTRUCTION 2 Sheeis-Sheet 2 Filed May 26, 1950 /nvenfor Geor e E. K/oofe Afforne United States Patent HOLLOW PANEL CONSTRUCTION George E. Kloote, Grand Rapids, Mich., assignr to Haskelite Manufacturing Corporation, Grand Rapids, lVIich., a corporation of lWichigan Application May 26, 1950, set-an No. 164,455

Claims. or. -35

This invention relates to hollow panel construction and more particularly to a hollow panel having a central core built up of arched ribs.

In the design and construction of hollow panels and particularly of hollow panels used as doors where they are subjected to extreme torsional, vibratory, shear and shock loads, it is essential that the design of the panel provide a proper combination of strength, warp resistance and dimensional stability. Numerous developments have been made in the design and construction of hollow panels having as their objective the development and combination, in a single unit, of these preferred characteristics. However, until the development of my invention, no entirely satisfactory design has been evolved, despite the fact that some of the proposed solutions involved the use of complex and expensive structures.

The endurance characteristics of a hollow core panel depend upon two important elements of its construction. The first of these elements is the design of the core and the second of these elements is the design of the panels or facing members mounted over the core. If either of these elements is weak or the strength and structural relationship of one of these elements to another one of them is not properly balanced, the resulting unit will not have the desired endurance and stability. It is not just enough to design the core and the facing panels to have great strength. In addition, there must be a balance between the relative strengths of various parts and a distribution of the strength throughout the various portions of the panel so that the panel, as an over-all unit, has substantially uniform resistance to the loads imposed. It is also essential that the panel have vertical and lateral stability against warpage due to torsional, shear and shock loads. Further, this resistance to warping, twisting or otherwise becoming misaligned must be capable of resisting not only the various operating loads but also loads created by variations in temperature and moisture. It is further important that the panels have dimensional stability, that is, neither swell nor shrink due to variations in humidity.

Besides the desired characteristics of strength, warpage resistance and dimensional stability, it is desirable that the panels have a constant thickness throughout and smooth surfaces in order that they may be suitable for fine finishes. Since these panels are normally used in places where their appearance is of considerable ornamental value, surface perfection is of utmost importance. When hollow core construction is employed, a waving of the surface panels between the points where they are supported and not supported is frequently encountered. The resultant wavy surface is unsatisfactory for varnish and stain coats as well as for painting. It is, therefore, essential that the exterior panels be so supported that the troughs and ridges, which may otherwise form, be broken up by the shape and design of the supporting members within the core.

Not only is the development of a Wavy surface a problem between the ribs of the core but also at the ice edge of the top and bottom rails. A troughing effect frequently occurs at these points due to accumulations of shrinkage stresses about the abrupt edge of these rails. I have found that by redesigning the edge of the rail in such a manner that these stresses are distributed over a greater area, this troughing effect can be either materially reduced or wholly eliminated.

Adequate ventilation of the interiors of hollow core panels is essential whereby moisture and temperature fluctuations in the ambient atmosphere will be permitted to cause similar and equal changes in the atmosphere trapped within the hollow core. Failure to take this precaution frequently results in extensive warpage not only of the panels surface but of its entire structure.

I have described the difficulties encountered in conventional hollow panel construction, particularly when these panels are used as doors. The occurrence of failure in hollow core doors is more frequent and noticeable than in other types of panels. However, the same difficulties, to a lesser degree, are experienced with all types of hollow panel construction. My invention contemplates improving the design and construction of all types of hollow panels, irrespective of their use.

It is, therefore, a primary object of my invention to provide a hollow core panel having a core of such design that it will resist all normal operating loads including shock and torsional twist and will resist warpage both vertically and horizontally.

It is a further object of my invention to provide a hollow core panel having dimensional stability.

It is an additional object of my invention to provide a hollow core panel on which the surface will be fiat, having no undesirable undulations.

It is a further additional object of my invention to provide such a hollow core panel which, although having great structural strength and stability, will be both light in weight and relatively inexpensive to fabricate.

These and other objects and advantages of my invention will be immediately seen by those acquainted with the art of hollow panel construction upon reading the following specification and the accompanying drawings.

In the drawings:

Figure 1 is a front, elevation view of my improved hollow core panel showing the surface panel removed to expose the interior core structure.

Figure 2 is an enlarged, fragmentary, sectional view of my improved hollow core panel taken along the plane IIII of Figure 1.

Figure 3 is an enlarged, fragmentary View of the top rail of my improved hollow core panel showing the mounting of the core ribs to the upper rail.

Figure 4 is an enlarged, fragmentary view of the center rail of the core of my improved hollow core panel.

Figure 5 is a front, elevation view of a modified construction for my improved hollow core panel showing the surface panel removed to expose the interior core structure.

Figure 6 is a front, elevation view of a further modified construction for my improved hollow core panel showing the surface panel removed to expose the interior core structure.

Figure 7 is an enlarged, fragmentary, front, elevation view of 1a modified design for the end rails for my improved hollow core panel.

Figure 8 is a sectional, elevation view taken along the plane VIIIVIII of Figure 7.

Figure 9 is an enlarged, fragmentary, front, elevation view of a further modified design for the end rails for my improved hollow core panel.

In executing the objects and purposes of my invention I have provided a hollow core panel having a core consisting of a single, centrally disposed rail dividing the core into an upper'and a lower section. Between the center rail and each of the upper and'lower rails are a number of individual, arched ribs; each one compressed along its longitudinal axis to form'a bow. The resulting core, enclosed by the upper and lower-railsand t h'estil'es, is

covered on each side by two or more lamina plywood panels or composition'boards to prbvide a smooth, flush surface.

In the following description, the terms upwardly and downwardly are frequently used and are to be taken as upwardly in the direction as shown in Figure l or as if the panel were mounted vertically in a wallor'hung as a door and the word downwai'dly as away therefrom. The terms inwardly and outwardly"" are also fre quently used 'andare to be taken asfinwaidly toward the geometric center of the panel and outwardly away therefrom. I 7

Referring now to the drawings ingrea ter detail, the numenal 1 indicates-a'peripherfl frame havinganfupper rail 2 and a lower rail 3 and stiles 4 ands. The stiles 4'and 5 are each securely fastened to the upper rail 2 and the lower rail 3 by staples, cement, glue or other appropriate bonding or fastening means to forma 's ecure'base frame for the core 6 of the door. Additional anchorage such as dowels or dovetails may be used where desired:

Midway between the upper rail 2 and lower rail 3 a lock block 7 is securely mounted to each of the stiles 4 and 5. A pair of lock blocks 7 are provided in order that the panel, when used as adoorgmay be hung from either side without creating di'fiicul ty in securing sufiicient support for mounting the lock hardware.

Midway between the upper rail 2 audlower' rail 3a center or intermediate rail '8'is securely mounted between the lock blocks 7. A notch'15 is provided in each of the lock blocks to seat the "ends of the i'nter'r'r'iedi ate rail 8.

The intermediate rail 8-is anchored to the lock blocks 7 by any suitable means such as stapling or cementing. The upper rail 2, lower rail 3, stiles'4 and 5 and the lock blocks 7 are each 'of thesam'e'thickness, but the intermediate rail 8 is of lesser thickness than these parts. The intermediate rail 8 is so-mounted to the lock blocks 7 that it is centered between the faces of the lock blocks whereby there is a step between each facefof' each of the lock blocks and each face'of the intermediaterail 8j 'for purposes which will appear more'fully hereinafter. The intermediate rail 8 is provided with a plurality of equally spaced, parallel, deep slots or channels 9 on one side and channels 9a on the other-side. The channels 9011 the upper side of the intermediate rail 8 are spaced midway between the channels 9a'of the lower side "of the interme'di'ate rail 8. Each of the channel's 9aj'nd 9a extends through the intermediate rail 8' substantially more than one-half the width of the rail'8 Wherebythe' blind'or lower ends of the channels-9 overlap'the blind cra er ends of the channels 9a. The purpose of this construction will appear more fully hereinafter.

The upper rail 2 :and the lower rail 13 are each provided with slots-10 spaced apart thesafne distance as the channels 9 and 9a. A groove 11 is provided in each of the upper rail '2 and-the lower rail 3, which groove 11 is deeper than the slots 10 but narrower than the upper rail 2 and lower rail 3 and centered "betweenthe faces of these rails. The slots 10' in both th upper rail 2 and lower rail 3 arevertical and perpendicular to the edge of these rails. The channels 9 and 9a and the slots,1 0 rareeach cut sufficiently wider than 'th'eribs that the ribs may be seated within them without"deflectin'g the ends of the ribs from the are assumed by the'rib (Fig. 4). Asfso seated, the e'r'id-bfthe rib contacts one wall'of the'slot slots 10, are pr'ovidedat the lower endoffeachfoffthe lock :7

blocks 7.

A plurality of ribs 20 are mounted between the upper rail 2'an'd'th'e intermediate-rail 8; Theupper end'ofeach of the ribs 20 is seated in one of the slots 10 in the upper rail 2 and the lower end ofeach of the ribs 20 is seated in one of the channels 9 on the upper side of the intermediate rail 8. Further ribs or rib elements 21 are mounted between the upper 'rail 2 and each of the lock blocks 7. These ribs have their upper ends in the slots 10 and their lower ends'in the slots 12 inthe lock blocks 7. Both the ribs 20 and the ribs- 21 consist of strips of rectangular cross section havinga longitudinal, cross sectional axis extending through..the thickness'ofthe panel. The thickness of the peripheral frame 1 and the length of the greater ofth'e cross sectional dimensions of the ribs 29 and 21 are equal. The ribs 20 are each designed to be longer than the spacing between the upper rail 2 and the intermediate rail 8 whereby when they are placed between these rails they must be longitudinally compressed in order to be seated; This compression bows them whereby they become an arch'of substantially constant radiu's't hrou'gh out their length. Each of the ribs 201's arche'd i'n the same direction, forminga series of substantially parallel'arches, each separated by a similarly curvedchamber or pocket extending between" the upper rail 2 and the intermediate rail 8. Similarly, the ribs 21are lon'ge'r than the distance between the upper rail 2' and the upper end of thelock blocks 7. Thus, they assume an arched shape when put in place. The ribs 21 are arched in the same direction as the ribs 20. 1 w 7 Between the intermediate rail Band the lower rail 3 are a further group of ribs 20a identical in design andinstallation to the ribs 20. Between the lower end of each of the lock blocks 7 and the lower rail 3 are ribs or rib elements 21a similar to the ribs 21. The ribs Zita have their upper ends seated in the 'channels 9a and their lower ends in the slots 10 in the lower rail 3. The ribs 21a have their upper ends seatedin the slots 12a and their lower ends seated in the slots 10 in the lower rail 3.

The ribs 20a and 21a are curved in a direction opposite to that of the ribs 20 and 21; By so arranging the'ribs, the intermediate rail '8 has no right hand or left hand form's and may be turned end for end or rotated about its longitudinal axis. Whether the ribs2ila an'clZla are curved to. the right and the 'ribs' Ztlan'd 2'1 are curvedto the left or vice versa does not affect the strength or stress distribution in 'my panel.

The frame 1 together with the intermediate rail 8; lock blocks '7, ribs 20, 20a, 21 and 21a, as a group, form the acre 6; Tothe core 6 there is mounted, on each side, a facing panel or sheet 25. Each facing sheet consists of a. "base lamination 26 and a surface lamination 27-. Whenthe lamination's' 26 and 27"are madeof a material having a distinct grain, such as wood, the grain of the base lamination 26' is positioned horizontally to the panel, that is, substantially at a right angle to the ribs 20 and 20a, and the surface lamination Z7 is positioned with its grain vertical to the panel and at a right angle to the grain of the base lamination 26. The facing sheets '25 may be made 'up"of more than "two laminates. They may also each consist of a single sheet of composition boardor a number of sheets of composition board. The construction of the facing sheets forms no part of my invention.

The various parts 'making'up the frame 1, core dandfa'cingsheets 25a're each generally designed to be made from wood. The grain ofthes'tile's 4'an d 5 and of the the 'facing sheets 25 are each designed-tohave their grain extending in opposite directions as described above.

The core may be constructed with the ribsabove'and below the intermediate rail each arched in the same direction (Fig. 5). The ribs 35 above'the intermediaterail 8 are identical to the ribs 36 below the intermediate rail 8. Similarly the ribs 37 between the upper rail 2 and the lock blocks 7 are arched the same way as the ribs 38 between the lock blocks 7 and the lower rail 3. The use of this particular arrangement is a matter of choice in fabrication rather than a structural modification affecting the strength and stability of the completed panel.

Assembly The stiles 4 and 5, the upper rail 2 and the lower rail 3 are each secured together to form a strong peripheral frame 1. They may be put together in any suitable manner as by stapling, gluing, cementing, with or without dowling or mortising. The peripheral frame 1 having been put together, the lock blocks 7 are mounted to one of each of the stiles 4 and 5 and the intermediate rail 8 stapled, cemented, doweled or otherwise firmly put into place between the lock blocks 7. The ribs and 20a are then put in place by having their ends seated in the appropriate channels and slots. The ribs 21 and 21a are similarly assembled. The base lamination 26 and the surface lamination 27 of each of the facing sheets are preassembled and the resulting assembly is then glued or cemented to the core 6. Preassembly of the facing sheets 25 may include either mere arrangement of the sheets in the desired order or preferably, in addition, a final bonding together of the sheets. The facing sheets 25 are bonded to the members of the frame 1, the lock blocks 7 and to each of the ribs 20, 26a, 21 and 21a by any suitable cement or glue, preferably a waterproof, resin glue. Normally this bonding is done under suitable conditions of heat and pressure. Thus, each of the various parts making up the panel is bonded to its adjacent part to form a rigid, structurally interlocked unit. To relieve the pressure of air trapped within the core 6 between the facing sheets 25, when the facing sheets are mounted, ventilating holes 30 (Fig. 1) are provided at each end of the panel. By means of the grooves 11 (Fig. 2) the air in each of the chambers or pockets 31 between the ribs may freely communicate with the holes 30. Since the thickness of the intermediate rail 8 is less than that of the members making up the frame 1, a passageway 32 (Fig. 2) is provided on each side of the intermediate rail 8. Thus, there is a free communication of air between the upper portion and the lower portion of the core and between each of the pockets 31 at the center of the panel. By means of the grooves 11 in the upper rail 2 and lower rail 3, there is also a free communication of air between the pockets 31 at both the upper and lower ends of the panels.

When a smaller panel 59 (Fig. 6) is to be made, such as a panel approximately equal to one-half the size of the panel shown in Figure 1, only one set of ribs is used. The panel 50 is provided with an upper rail 51 and lower rail 52 similar in construction to the rails 2 and 3. The rails 51 and 52 are joined by stiles 53. The ribs 54 extend between these rails 51 and 52 and the intermediate rail is not used. The ribs are arched by compression in the same manner as the ribs 20. The ribs may be arched to the left or to the right. When the lock blocks are dispensed with, the resulting gap between the stile and the concave side of the first adjacent rib 54 is filled by means of a special rib 55. The rib 55 is shorter than the ribs 54 and is arched in such a manner that it is substantially parallel to the ribs 54. The ends 56 of the rib 55 are stapled, nailed or otherwise suitably fastened to the stile. The use of the special rib 55 eliminates the gap of excessive width which would otherwise exist in the area and prevents undesirable stress concentrations.

Operation By arching each of the ribs forming the central structure of the core 6, the ribs are given greatly increased strength to resist twisting about their central, longitudinal axis. Their resistance to bending or collapsing under compressive loads applied against the surfaces of the facing sheets 25 is greatly increased. The combination of the rigidly bonded ribs and surface sheets forms in tegral, structural unit. In this structure the facing sheets support the ribs against pivoting about their ends, thus giving the entire panel strong resistance to bending or warping. In order to curl a hollow panel having a rib structure it is necessary to bend or flex the ribs within the panel. The combination of the arched or curved ribs with the facing sheets provides a structure particularly adapted to resist such warping or bending.

By arching the ribs, each rib supports a greater area of the facing sheets. The ribs are each curved sufliciently that a vertical line drawn between the upper and the lower ends of one of the ribs will intersect a substantial segment of the next adjacent rib. Therefore, each vertical segment of the surface sheets 25 is supported by two ribs, the same rib at each end and a different rib at the center. Thus, each vertical segment is alternately supported and not supported as it crosses portions of the two ribs involved. Since the grain structure of the facing sheets 25 is, in the base lamination, horizontal to the panel and, in the surface lamination, vertical to the panel, the arched ribs are neither parallel nor perpendicular to the grain of either of the laminations making up the facing sheets 25. The stress lines set up in the laminations of the surface sheets 25 and in the ribs, when the panel is subjected to a load, are not parallel in the core structure and the surface sheets. Therefore, there is no tendency for the stresses to concentrate in either the surface panels or the ribs. This adds materially to the strength of the door in all directions and reduces the possibility of local failures. The surface sheets are completely and effectively supported against ridging or the formation of undulations.

In addition to the structural advantages obtained from the arched design of the ribs, the design of the intermediate rail 8 contributes materially to the strength and appearance of the panel. Since the intermediate rail does not, at any point, touch the facing sheets 25 there is no bond between these parts. Thus, there is no solid, continuous support for the surface sheets established across the middle of the door, which support would normally be parallel to the grain structure of one of the laminations of the facing sheets 25. Where such a support is provided, unequal stress concentrations are set up due to the provision of a rigid anchorage along a narrow band extending across the center of the panel. By designing the intermediate rail 8 in such a manner that the ends of the ribs 20 and 20a overlap traversely of the panel, a complete and adequate support is provided for the facing sheets 25 without the necessity of providing a continuous solid support. Since the ends of the ribs 20 and 20a overlap, the grain lines of the base lamination 26, in the area of the overlap, contact the ends of every rib. Thus, the line of stress concentration which is set up by the ends of each of the groups of ribs is not a straight line parallel to the grain but a wavy line crossing numerous grain lines. This structure prevents any drawing in or undulation of the facing sheets 25 in the area of the intermediate rail. Further, the intermediate rail is left free to pass the stresses from the ribs 20 to the ribs 20a without interference from the facing sheets 25. This gives the entire panel stability to resist warping and to readjust its interior stresses. These stresses will not become concentrated at the center of the panel to create local distortions. Further, by means of the passageways 32 between the intermediate rail 8 and each of the facing sheets 25, air may flow freely from the hole 30 at one end of the panel throughout the whole central core of the panel and out the hole 30 at the other end of the panel. The holes 30 are left open to permit this air flow, for balancing the barometric pressure and humidity content of the interior of the panel with that of the ambient atmosphere.

By using the arched rib construction, the effect of a lattice work or cross-hatched type interior core struc- 7f ture isneffected without the stressconcentrations attendant such lattice typeconstructionsand Without the necessity of providing intersecting. ribs The ribs parallel each other rather thanintersect, Since there is no intersection of the ribs, it is unnecessaryto;weal enthe ribsby the slots and channels conventionally'used in; lattice work designs. Yet, at the same time, all of theadvantages ofhaving the grain 3 of; the ribs extend in; a different direction from that of the laminations of the facing sheets is obtained Further, the-fabrication and assembly of ribs with numerousslots and;c uts is expensive, all; of which os re m nawd1 h des ssioftmy mp ov -co ee e e. Pan 1 si ne spe ifi al y o al -S in which no hardware is goingto be employedg such as a wall P lx hP- Q KLUQPK m yim z fil t t. h nblocks 7 are eliminated, the; intermediate .rail 8.is attached directly to thest-iles 4 and -tand;theribs21 and 21aare eliminated and additional r 2t]. and -2Qa;are substituted.

the lock lbloclgsfl are; dispensed within the larger p n l tt ha fl akshcw issur sh a pe ia r r ilar-to the rib;55 (Big-,6) .is used in the resulting -ga1: .;b e tween the :concave; side.- of;the ribs 20 s and-1 20a; and the stiles 4: and 5,,resp. ectivel y. This -modifieationdoes. not in y a afiect he rwrat c ali d e i n' p c p of my panel. It is -merelyi a modification of the application of:these; principles. It is possible-to add-a dditional larninations to the facing; sheets wheresuchis desired. The .panelmay be .constructedinany-onebf a number of suitable. thicknesses and shapes, depending upon the requirements of;the use to which the panel is to be \put.

In order to eliminate troughing of the surface sheets adjacent the upper and lower rai1s,. these; rails may be specially designed to disperse the stresses concentrated in this area. One -way., of doingthisjs to provide therail 60 with;a.kerf;6 1- (Fig, 7' and;8=) on eachside of, parallel to and adjacent the inward face,62 of the rail, The kerfs 6l.are close enoughtotheinward face 62. of the rail 60 and deep enoughthat the flanges-63 created between the kerfs and the, .face 62 may -deflectslightly in response to stresses originating in both the rail; andthe facing sheets. The; deflection; of these flanges relieves these stresses distributing them over an, appreciably greater areawhereby theywill not ;cause.;loc al troughing of the facing sheets at =the edges of the. rails,

The ribs 64 :aremounted, ;to-;the ,rails in the same rnan-v ner as is shown; in Figure 3.; The. Slots65; for theribs 64 penetrate therail beyondthe kerfs 61. Since-a groove similarto thegroove 11 cannotbe used-in conjunction with the kerfs 61; air circulation is ,provided for by means of the holes 66= througlreachof the-.ribsfimadjacent the.

Material relief of; stresses. adjacent the edge of l the. rails may also beaccomplishedby scalloping the inward face 71 of; the trail 70;;(Fig. 9) ,to'produce smoothly rounded,; alternate ridges, 72; and valleys 73.. This scall-opededge distributes the-stressesin such a manner that.

they cannot;concentrate-alongany one particular line, pa ticularly a line parallel to thelgrain. of any ofthe. laminations of the facing sheets.- Itis impossible for a continuous trough-to form becausethe undulating edge line of the scalloped; rail breaks up these stress concentrations. The .slo ;74,for thev ends of the ribs. '75:.are preferably located at the crown ofeach loflhe ridges 72;

Althoughthe ribs1may be attached to therai-lsat the center of the valleys 73yor. between the valleys i73 and the. ridges 72, the location of thet slots 74 for the railsv 75 at the crown of the ridges .72 reduces the depth to which the slots 74 penetrate the, rails, thus materially increasing the strength ofthe rails, It is important that each of the slots be located at the same point with respect to the ridges andvalleys in order that all ribs will-be ofthe same length: A groove similar to the groove 11 deeper than the slots 74, may be usedto permit cross ventilation between the'pocket'sdefined by the ribs. If: such a groove is not used, hol es: should be I provided; threugh each-of the ribs. similar to the holes 66;

h w fin Fi ur I 'have described an 7 improved design for; the construetion;of a hollow; core'panel.

may be made ofimydmprovedpanel; eachof which will v not depart frorn the basicprinciples herein: disclosed.

Each of these modifications is to be considered in the hereinafter appended claims unless these claims by their language expressly; provide otherwise.

Iclaim:

1; In .ahollowpanel having, a core structure andenclosing sheets. on each. face, the improvement; in. said corecomprising; aperipherali frame including spaced stiles, a first end rail and.;a secondend'raihsaid stiles being of substantially greater-rlengththan'said endrails;

one wall of each of said firstend rail andsaid secondend' raildefininga plurality of spaced slotsyan intermediate rail mounted tosaidpen'ph'era-l frame. and spaced substantially fromtboth said firsthendarailand said second end: rail; the Walls of said intermediate rail: defining a firstgroup of spaced slots and a second groupof spaced slots; said first-groupoftslots oppositely directed from said; second group of; slots; a first. plurality of spaced, substantially parallel, arched ribs each seated atone-,ofzitsends inone ofthe slotsin said first end rail andtat the. other of its ends seated in .oneof the slots of said first groupflofslots in said intermediate rail; said first plurality of ribs arched toward one. of said stiles; a second plurality of-spaced, substantially parallel, arched ribs each seated at one ofzits ends in one of1the slots in said second end rail'andat the other of its ends. in

one of; theslots of -said second group of slots in saidintermediate rail; said second plurality of ribs. arched toward one of'said stiles.

2. Inahollow panelas described in claim 1 wherein said intermediate rail'has a:thickness substantial1y less thanthe, thickness of said stiles; said intermediate rail spaced from one of said facing sheets.

3. Inahollow panel ascdescribed-zin claim. 1 wherein said interrnediate rail has .athickness substantially less thanthe thickness .of said stiles; said intermediate rail spaced from eachof'said facing. sheets for defininga passagewaybetween said intermediate rail and each of said facing sheets.

4.- In avhollow panel; having a pair of spaced facing sheets, a plurality of internalribs, an intermediate rail comprising: a beam ofsubstantiallylesser thickness thansaid spacing between said facingsheets; thewall On one side of .said beam defining a vfirst plurality of spaced slots each open at one; of its ends; the wallon the opposite side of said intermediaterailgdefining-a second plurality of spaced slots each open at one .of its ends; the open ends of said first plurality of slots directed oppositely to---the open ends-0f said. secondplurality of' slots; said first-- plurality of'slots spaced along said beam intermediatesaid second plurality of slots; each of said slots in said-- beam adapted .to receive. one end of one of said internal ribs; each of saidrslots ofsaid-first plurality of-slots' stantially from both said first end rail and said second end rail, the walls .of saidintermediate rail defining a first group off space d slots and a second group of spaced slots; said firstgroup of spaced slot oppositely directed Numerous modificatjons:

from said second group of spaced slots; each slot of said first group of slots and of said second group of slots penetrating said intermediate rail more than one-half its width; a first plurality of spaced, substantially parallel, arched ribs each seated at one of its ends in one of the slots in said first end rail and at the other of its ends seated in one of the slots of said first group of slots in said intermediate rail; said first pluarlity of ribs arched towards one of said stiles; a second plurality of spaced, substantially parallel, arched ribs each seated at one of its ends in one of the slots in said second end rail and at the other of its ends in one of the slots of said second group of slots in said intermediate rail; said second plurality of ribs arched toward the other of said stiles.

References Cited in the file of this patent UNITED STATES PATENTS 564,780 Cordray July 28, 1896 10 Shelton Aug. 18, Ripper Nov. 6, Johnson May 12, Thomas May 20, Pretot Feb. 22, Weyerhaeuser Aug. 1,

FOREIGN PATENTS Great Britain Aug. 25, Switzerland May 31, France June 24, Sweden July 9, Denmark May 9, Norway Mar. 28,

OTHER REFERENCES American Builder, volume 71, page 134, November 

